Your and My Secret (2005)

Original title: 僕と彼女の×××

The story revolves around high school student Akira Uehara, sensitive and demure by nature. Despite that he has a large crush on the most feared and violent girl in school-Nanako Momoi, who seems fragile and cute, until she opens her mouth. One day, Uehara is made to go to Momoi's house where he meets her mad scientist grandfather and unwillingly becomes the test subject for one of his machines. Accidentally, Momoi alsogets tangled in the experiment and the two end up switching bodies. To make matters worse Momoi in Uehara's body breaks the machine (which will cost 2mil. yen to fix) as she would like more time in this body thus beginning their world full of debts and troubles.